1. Field of the Invention
An improved method for processing shallow-water marine seismic data that has been contaminated by noise transients such as environmental noise generated by shrimp, croaker fish, whales and the like.
2. Description of Related Art
During the course of a marine seismic survey, a long string of several hundred seismic sensors is towed through the water by means of a suitable cable. Periodically, such as every six to eight seconds, an acoustic source is triggered to emit a wavefield to insonify the subsurface earth layers thereby to generate a reflected acoustic wavefield that is detected by the sensors. The detected mechanical signals representative of the reflected wavefield are converted to electrical signals by the sensors. The electrical signals are transmitted through conductors in the cable to the towing ship where the signals are recorded for future analysis to display the topography of the subsurface earth layers. The seismic signals from each source-triggering are digitized and recorded in the form of a set including a plurality of discrete time series, one time series corresponding to each sensor or sensor group connected to the cable. The set of discrete time series resulting from each source-triggering may be displayed as a single multi-trace visual seismic record. The traces may take the form of variable amplitude or variable density or variably colored traces.
In shallow water at depths on the order of 20-250 feet, so-called bay cables are used. For certain technical reasons, a typical bay cable includes co-located sensor pairs of different genera. Usually a pressure sensor is paired with a gimbal-mounted particle-velocity sensor, both of which sensors are customarily laid directly on the sea floor. Shallow water operations are so well known and routine to those skilled in the art of geophysical exploration that a prolix explanation and accompanying illustrations should not be needed.
Any unwanted wavefield that appears on a seismic record is considered to be noise. Many different sources create the noises that afflict the seismic signals received during the course of a shallow-water marine survey. Whereas whale-talk is bothersome noise to the geophysicist, it may be music to the ears of an environmentalist.
Reverberation due to acoustic signals bouncing between the sea floor and the water surface seriously distorts the desired reflected wavefields. Various methods are known for eliminating such contamination such as U.S. patent application Ser. No. 08/101,949, now U.S. Pat. No. 5,365,492, issued Nov. 15, 1991 to W. H. Dragoset and assigned to the assignee of this invention. That patent describes a method for scaling the outputs from sensors of different genera relative to each other. Additionally, an adaptive noise cancellation process was taught for reducing the noise content of the respective signals prior to summation.
Seismic shots generated by a rival seismic crew produces coherent noise. U.S. Pat. No. 4,937,794, issued Jun. 26, 1990 to R. A. Marschall et al. explains how coherent noise on seismic records may be suppressed by formatting the common shot gathers to common receiver gathers. Pairs of seismic traces from a common receiver gather are corrected for differential normal moveout, weighted in inverse proportion to the RMS signal power, and combined to generate compressed common receiver gathers. The compressed common receiver gathers may be reformatted as common midpoint gathers for further processing.
Noise due to near-surface scatterers may be removed or minimized in accordance with the teachings of U.S. Pat. No. 5,293,352, issued Mar. 8, 1994 to Ronald E. Chambers and assigned to the assignee of this invention. Therein it is taught that a set of raw common shot gathers are resorted to common receiver gathers. The wavefield envelopes from the common receiver gathers are migrated using one-half the near-surface velocity to provide migrated data sets. The migrated data sets are resorted back into common shot gathers and subtracted from the original raw common shot gathers to provide coherent-noise reduced data sets.
A method that was designed originally for use with vibroseis data on land but one that could be adapted for use in shallow marine projects explains that a reference model of the level of a valid seismic signal is built for each of a number of time windows during a seismic data recording cycle. The model is selected by obtaining the average of the absolute magnitudes for each time window from a number of sweeps to form a set of averages. The median of each set is selected and is padded by a suitable coefficient to provide the reference model for each time window. Subsequently a recording is made. Each data sample from the recording is compared to the reference model corresponding to the time window that includes the sample. If the amplitude level of the data sample exceeds the reference model level, the sample is suppressed prior to summing. See U.S. Pat. No. 4,344,158, issued Aug. 10, 1982 to Ralph A. Landrum, assigned to the assignee of this invention.
A particularly troublesome type of noise, that is not easily handled by known methods, is biological. FIG. 1 illustrates an example of such noise which is believed to have been due to a Grand Betail or to croaker or similar fish. The noise bursts are the inverted arrow-like diffraction patterns such as 10. The desired wavefields are the broad gently-curved hyperbolic waveforms generally indicated by 12.
There is a need for a noise-abatement technique for attacking unwanted singular transients.